And make constants P for which we do not want of_val P to reduce Opaque.

iSpecialize and iDestruct.

These tactics now all take an iTrm, which is a tuple consisting of
a.) a lemma or name of a hypotheses b.) arguments to instantiate
c.) a specialization pattern.

This gets rid of the (ambiguous) notation %l, because we can declare
LitLoc as a coercion. It also shortens the code.